Projection systems require a high quality light source. While projection systems have traditionally used discharge lamps as a light source, there is now interest in alternative light sources such as lasers. Lasers have several advantageous properties. They emit a high intensity light beam and have a very long operating lifetime. However, laser light sources also have some disadvantageous properties. A laser light source emits a narrow spectrum of coherent light and projection systems with laser light sources can suffer from an effect called “speckle”. Speckle is typically seen as a granular structure on the screen which deteriorates the image quality and is distracting to a viewer.
It is desirable to reduce speckle in order to obtain an acceptable image quality from a laser-based projection system.
US 2010/0165307A1 describes a laser image display system with reduced speckle. The display system comprises a laser light source and an optical integrator. The light source comprises a single red laser, a single green laser and a single blue laser. Light from the lasers is output, via a beam deflector, to an optical integrator. The optical integrator includes, part-way along its length, an element which diffuses light. This system uses time averaging of speckle patterns resulting from different amounts of deflection of the laser beam. A drawback of this approach is that different angles are generated sequentially by controlling the motion of the beam deflector. This poses a serious eye-safety risk at high projector brightness as all the light at a certain moment in time appears to originate from a single spot in the projection lens aperture. Furthermore, negative effects are likely to occur when used with certain types of light modulators that are driven by pulse width modulation (e.g. micro-mirror arrays). Aliasing effects between the PWM scheme and the beam deflector motion could occur unless both are synchronized. But if synchronized only specific deflection angles will be sampled by the light modulator and limited speckle reduction will be achieved.
US 2009/139865 proposes the combination of laser array light sources with overlapping circular Gaussian beam profile in the far field, a single integrator and a temporally optical phase shifting device. The disadvantages of this approach are the long optical path length and the precise alignment required to fulfill the overlapping far field condition.
In the book of J.W. Goodman, “Speckle phenomena in optics: theory and applications”, Roberts and Company Publishers (2007), three fundamental methods are identified to reduce speckle inside the projector. Namely: angular diversity, wavelength diversity and polarization diversity.
The use of a beam deflector or optical phase shifting device both result in speckle reduction by angular diversity. However for practical high-end projection systems this approach only cannot deliver the desired level of speckle reduction. The size and acceptance angle limits of commercial light valves restrict the amount of angular diversity that can be introduced. In US 2009/139865 the combined use with polarization diversity is suggested, but it is recognized that for stereoscopic projection based on polarization this method cannot be used.